There have conventionally been known an imaging apparatus and radiographic apparatus using a sensor array which adopts, as a material, amorphous silicon or polysilicon formed into a film on a glass substrate and is constituted by two-dimensionally arraying photoelectric conversion devices and TFTs. These apparatuses generally transfer charges photoelectrically converted by the photoelectric conversion device to a readout apparatus and read them out by matrix driving using the TFT.
The circuit configuration of the conventional imaging apparatus will be described. FIG. 5 is a schematic circuit diagram showing the circuit configuration of the conventional imaging apparatus.
As shown in FIG. 5, the conventional imaging apparatus has a sensor array in which pixels made up of PIN photodiodes S11 to S33 and thin film transistors (TFTs) T11 to T33 of amorphous silicon are arrayed, and performs matrix driving. The common electrode sides of the PIN photodiodes of the pixels receive a bias voltage Vs from the power supply. The gate electrodes of the TFTs of the pixels are connected to common gate lines Vg1 to Vg3, and the common gate lines Vg1 to Vg3 are connected to a gate driver 2 having a shift register (not shown) and the like. The source electrodes of the TFTs are connected to common signal lines Sig1 to Sig3. Signals output to the common signal lines Sig1 to Sig3 are output as digital image data to a digital output bus via a readout apparatus 101 having preamplifiers 16, an analog multiplexer 11, an A/D converter 13, and the like. In general, the analog multiplexer 11 is constituted by a plurality of amplifiers, analog switches, shift registers (none are shown), and the like.
The sectional structure of the pixel of the sensor array will be explained. FIG. 6 is a sectional view showing the pixel of the sensor array.
In each pixel, a gate electrode layer (lower electrode) 202, an insulating layer (amorphous silicon nitride film) 203, an amorphous silicon semiconductor layer 204, n-type amorphous silicon layers 205, and source and drain electrode layers (upper electrodes) 206 are stacked on a glass substrate 201 to constitute a selector thin film transistor (TFT) 222. Also, an extending portion (lower electrode layer) of the source/drain electrode layer 206, a p-type amorphous silicon layer 207, an amorphous silicon semiconductor layer 208, an n-type amorphous silicon layer 209, and an upper electrode layer 210 are stacked on the glass substrate to constitute a photodiode 221. The glass substrate 201 also supports a wiring portion 223 constituted by stacking the insulating layer 203, amorphous silicon semiconductor layer 204, n-type amorphous silicon layer 205, and source/drain electrode layer 206. A protective layer 211 is formed from an amorphous silicon nitride film or the like to cover these layers, and a phosphor layer 213 is adhered onto the protective layer 211 by using an adhesive layer 212.
The phosphor layer 213 is arranged to convert radiation (X-rays) into visible light. A photodiode formed using amorphous silicon generally exhibits low sensitivity to X-rays. The phosphor layer 213 is formed from a gadolinium-based material, CsI (cesium iodide), or the like.
In the conventional photoelectric conversion apparatus (radiographic apparatus), when X-rays having passed through an object are incident on the phosphor layer, they are converted into visible light, and visible light is incident on the photodiode. In the photodiode, charges are generated in the semiconductor layer, when the TFT is turned on, sequentially transferred to the readout circuit, and read out.
The operation of the conventional imaging apparatus will be described. FIG. 7 is a timing chart showing the operation of the conventional imaging apparatus.
First, the preamplifiers 16 and common signal lines are reset by a reset signal RC from a timing generator (not shown). Then, a pulse is applied to the common gate line Vg1, and the TFTs T11 to T13 connected to the common gate line Vg1 are turned on. Signal charges generated in the photodiodes S11 to S13 are transferred to the readout apparatus 101 via the common signal lines Sig1 to Sig3. The transferred charges are converted into a voltage by the preamplifier 16. A sample-and-hold signal SH is applied from the timing generator (not shown) to the readout apparatus 101, and the voltage output from the preamplifier 16 is sampled in a sample-and-hold capacitance.
The voltage sampled in the sample-and-hold capacitance is serially converted by the analog multiplexer 11, and output to an analog data line. The serial analog signal output to the analog data line is input to the A/D converter 13, A/D-converted by the A/D converter 13 in synchronism with a clock signal AD_CLK, and output to a digital output bus corresponding to the resolution of the A/D converter 13. When the conventional imaging apparatus is applied to a medical radiographic apparatus, the resolution of the A/D converter is generally 14 bits or more.
This operation is repeated for the gate lines Vg2 and Vg3, completing readout from a whole sensor array 4. Note that incident light (or X-rays) may be continuous light (or continuous X-rays) or pulsed light (or pulsed X-rays).
The conventional imaging apparatus which reads out signals by matrix driving from the area sensor array prepared by arraying photodiodes and TFTs has simply been described.
Other prior arts regarding the area sensor array structure and matrix driving are disclosed in Japanese Patent Laid-Open Nos. 8-116044 (refer to corresponding U.S. Pat. No. 6,075,256 B1) and 11-331703 (refer to corresponding U.S. Pat. No. 6,185,274 B1) and the like. The detailed configuration and operation of the readout apparatus are disclosed in Japanese Patent Laid-Open Nos. 2002-199292 (refer to corresponding U.S. Pat. No. 6,538,591 B1) and 6-235658 (refer to corresponding U.S. Pat. No. 5,448,056 B1) and the like. The imaging apparatus which matrix-drives the area sensor array and reads out data as a digital output by the readout apparatus including the A/D converter is disclosed in Japanese Patent Laid-Open Nos. 9-307698 (refer to corresponding U.S. Patent Application Publication No. 2001/0012070 A1) and 11-150255 (refer to corresponding U.S. Pat. No. 6,798,453 B1) and the like.
In the conventional imaging apparatus, however, thermal noise of the preamplifier 16 arranged in correspondence with each signal line and the like may increase to non-negligible level and influence the noise characteristic. Similarly, thermal noise of the amplifier arranged at the output of the analog multiplexer 11 and the like may increase to non-negligible level and influence the noise characteristic of the imaging apparatus. This state is shown in FIG. 8. That is, in the conventional imaging apparatus, the S/N ratio of the whole imaging apparatus may decrease owing to thermal noise of the amplifier which constitutes the readout apparatus 101, that of the resistor used at each portion, and the like.
Regarding this phenomenon, Japanese Patent Laid-Open No. 9-307698 describes reduction of thermal noise, i.e., KTC noise caused by the ON resistance of a switch used for a preamplifier. Japanese Patent Laid-Open No. 9-307698 describes an LPF means arranged in correspondence with each signal line in order to reduce high-frequency noise generated by thermal noise of the preamplifier or the like. Japanese Patent Laid-Open No. 9-307698 also describes a function of changing the timing of KTC noise cancellation and the through rate of the amplifier in order to switch between moving images and still images.
However, it is difficult to satisfactorily reduce noise even by the method described in Japanese Patent Laid-Open No. 9-307698 or the like.